hey guys I was perusing Craigslist and found a large quantity of commercial metal halides. They are 400W hanging fixtures. Is there any difference between these and standard aquarium metal halides?. Obviously the reflectors are different but could these work?

The ballast and mogul bases will work fine. Just plan on a big electric bill if you use 400w MH's. Not sure about the "but save big money and get big watts" thing. Dunno what they cost on CL, but you can get a 400w magnetic core & coil ballast, same as in those fixtures, for like $40. They aren't expensive.